(a) Field of the Invention
This invention relates to an improved process for producing carbon fibers and, more particularly, for producing carbon fibers in which solvent refined coal (SRC) obtained by heating coal under hydrogen pressure in a solvent is used as the starting material.
(b) Description of the Prior Art
Processes for producing a carbn fibers such as carbon fibers and graphite fibers can be divided into two main classes with respect to the starting material employed: one is a process in which a natural fiber or a synthetic fiber is used as starting material and another is a process in which petroleum pitch or coal-tar pitch is used as the starting material.
The former has the disadvantage that a natural or a synthetic fiber, such as polyacrylonitrile and the like is a very expensive raw material and that the carbonization yield of the fiber is rather low. On the other hand, the latter process requires a heat treatment of the pitch for improving spinnability and for rendering the fiber non-fusible. It is very difficult to maintain uniformity in the properties of carbon fibers prepared from petroleum pitch and coal-tar pitch, due to the fact that neither petroleum pitch nor coal-tar pitch is originally uniform and that heat treatment thereof results in considerable nonuniformity in composition. For example, heat treatment of a petroleum pitch causes pyrolysis and condensation polymerization at lower temperatures thus creating difficulty in the preparation of a pitch with the preferred properties. The starting material should have a softening point in the range of from about 150.degree. to 250.degree. C. and a high carbonization yield.
In order to overcome the difficulties of the prior art processes, it is necessary to adjust the properties of the starting material by subjecting these pitches to a heat pre-treatment, but the pre-treatment makes them non-uniform in their compositions. Further, the use of a highly aromatic starting material such as coal-tar pitch, petroleum pitch or the like has the disadvantage that the time required for making them non-fusible is prolonged, because the speed of oxidation of an aromatic ring is lower than that of an aliphatic chain or of an alicyclic ring. Accordingly, when coal-tar pitch or petroleum pitch is used, the conversion of an aromatic ring into an alicyclic ring by hydrogenation is conducted in advance. Further, in a process in which these pitches are used as starting material, the heating to render non-fusible a spun fiber must be effected at a rate of from 0.5.degree. to 1.degree. C./min, unless the fiber is subjected to a pretreatment with ozone, because higher heating rates than the above range cause fusing between the fibers.